Add show jobs

[joaander]

Description

Add the subcommand show jobs.

Proposed solution

Like show directories, show jobs will write one table row per cluster job that row is currently aware of. For each job, write the action it is running and the number of directories. We could also list the directories the job is running, up to a maximum provided by the user.

The --short option (#21) will list only the job IDs.

Additional context

Users will find show jobs helpful to identify which job is executing for a particular action/directory (e.g. row show jobs -a action1 directory2).